In a survey of invasive streptococcal infections in Japan, we analyzed isolates of Streptococcus pyogenes collected between 1992 and 1994. Genomic DNA fingerprints produced by pulsed-field gel electrophoresis (PFGE) were compared by computer-assisted analysis. Among the 42 isolates from patients with invasive diseases, 16 PFGE-types were identified and genetic diversity was clearly demonstrated. Identical fingerprints were observed in both invasive and noninvasive isolates. Only 43% of invasive isolates produced streptococcal pyrogenic exotoxin A (SPE A) and 31% did not contain the spe A gene. These findings suggest that the dissemination of a specific clone is not sufficient to explain all cases of these diseases in Japan, and pose a question to the role of SPE A as a major virulent factor.We have also observed 4 cases of transmission of this organism among a patient with invasive disease and his or her family members. The clonal spread among the family members was confirmed by analyzing genomic DNA fingerprints of the isolates recovered from them. Although 14 persons resulted in only colonizers or self-limitted diseases, 3 developed invasive infections. Our findings indicate the need of antibiotic chemoprophylaxis for close contacts of patients with invasive streptococcal diseases.